The invention relates to a method for producing boards of wood-based materials with structured surfaces as well as smooth surfaces, such as particle boards/fiber boards and chipboards and also synthetic boards comprising a combined component of wood and plastic or composed of plastic with and without reinforcing layers with structured surfaces, as well as a plant and a continuously operating embossing press for carrying out the method.
A method and plant of this kind are disclosed in DE-P 197 18 771.4. The object of that patent application is to produce boards of wood-based materials with or without an interruption in production, the mechanical strength properties of the finished boards and adhesive strength on the structured side being unimpaired.
The solution specified for the method is that the board of wood-based material just finished and released from the pressing operation is sprayed with water on one or both surfaces, after which it passes through a heated steam treatment section and is then subjected to a structuring process.
The plant for carrying out the method according to claim 6 of DE 197 18 771.6 for a continuous process consists in that a water spray device, followed by a heatable or unheated steam hood and then a structuring roll frame designed for one or both surfaces are arranged transversely across one or both surfaces immediately after the point where the board of wood-based material emerges from the press for a continuous process.
However, the method and plant in this prior application do not reveal or indicate any way by which process parameters for pressure and heat are to be used and, if required, in what time sequence the embossing pressure and heat input are to be used and how a plant and, where applicable, embossing press suitable for this must be embodied. In the case of a continuous process, it is namely not possible to introduce a structure by means of an embossing device without suitable and coordinated surface dampening and a subsequent appropriate plasticization process. This means that if the embossing pressure is not suitable and the internal temperature and moisture content of the boards is not sufficient, it is not possible to achieve adequate long-term pattern accuracy from the mechanically applied embossing pressure on the cover layers. Even after a prolonged embossing time, for example about 15-20 seconds, and at an embossing pressure of 7 N/mm2, the embossed particle/fiber structures spring back into the state of a smooth surface, i.e. the embossed pattern disappears as soon as it leaves the embossing zone that imposed the structure.
Swiss Patent 614 666(=U.S. Pat. No. 4,007,076) has disclosed a method for embossing a synthetically produced board, primarily a hardboard. Here, water is applied to the surface of a cold board. However, this results in long process times because the water first has to be raised to the vaporization temperature. The quantity of surface water used, 54 g/m2, is too low for the boards to be embossed in accordance with the present application to ensure sufficient plasticization in a short time. The inadequate surface moisture requires excessive pressures and an over-long total pressing time for permanent pattern accuracy of the embossed structure. Overall, this method does not provide any suggestion or impetus as to how permanent structuring of a board that has just emerged from the production press and is still hot is to be performed.
An object of the present invention is to indicate a method and create a plant with a continuously operating embossing press for imparting a permanent structure to the surfaces of a still hot board after it leaves a production press.
Another object of the present invention is to create board production plants operating to create both boards with and without structured surfaces.
A further object is to provide a flying changeover during the transition from a smooth to a structured surface, without the production of reject finished boards and without trimming waste, i.e., waste caused by cutting out sections of lower surface quality, and in small batch sizes for order-based production quantities.
Still another object is to create a means of controlling the production process so that there is no transition zone with an undefined surface quality between the smooth and the structured section of the board.
Another object of the present invention is to control a transition-free selection of the surfaces virtually on a numerically recorded line and with on-line production changeovers being technically possible.
Further advantageous measures and configurations of the subject matter of the invention will become apparent from the following description together with the drawings.